Skipping `get_spans` in GreedyCoverage aligners

This new version adds:

• The possibility of skipping the get_spans step in GreedyCoverage aligners, when passing the spans already computed in token_to_chars by HuggingFace tokenizers. This result in faster execution of the algorithms.
• Reran the benchmark to depict the time changes.
• Extended the explanation of GreedyCoverage aligners in the README.